The speaker unit includes a so-called dynamic type speaker unit including, for example, a magnetic circuit constituted of a magnet, a yoke, and a coil (voice coil). In this dynamic type speaker unit, a coil bobbin wound with a coil is axially vibrated for outputting sound.
As this speaker unit, there is a type that is operated by voltage drive. The driving force for the speaker unit is proportional to the current. Therefore, in the case of the voltage-driven speaker unit, there is a fear that a linearity between the voltage and the driving force breaks down in various situations and the quality of sound output from the speaker unit is deteriorated.
For example, in the voltage-driven speaker unit, it becomes more difficult for the current to flow through the coil as the frequency domain becomes higher. Thus, the output is lower in a high-frequency domain.
By the way, a current-driven amplifier exists. When the speaker unit designed to be driven by the voltage is operated by this amplifier, a magnetic brake due to power generation of the coil is cancelled. Thus, large oscillations are generated due to spring vibration at a frequency near the minimum resonance frequency, which deteriorates the sound quality.
In view of this, there is a speaker unit that is adapted to reduce oscillations at the frequency near the minimum resonance frequency by attaching a resistance-adding seat such as a non-woven sheet that adds an air resistance to the back pressure.
However, in such a speaker unit, the number of parts is increased due to the provision of the seat, which causes a problem of deterioration of the outer appearance as well as an increased manufacturing cost and an additional working process of attaching the seat. Further, if the seat is separated, the function of reducing the oscillations is deteriorated.
From these perspectives, there is proposed a speaker unit in which a magnetic fluid having a viscosity that provides a suitable brake is injected into a magnetic gap formed between a magnet and a yoke (e.g., see Patent Documents 1 and 2).